Bacteriophage Phi6 contains three pieces of double-stranded RNA in its nucleocapsid. The pieces contain different genetic information but have similar termini. The aim of this proposal is to determine how the packaging system is able to select one of each of the chromosomes. Specifically, the regions of the three chromosomes necessary for packaging will be identified. The order of packaging and the requirements for packaging in terms of amount of RNA will be determined. The studies will depend upon the use of cDNA clones of the Phi6 chromosomes. Chimeras and other constructions of the cDNA clones will be prepared and the packaging of the transcripts of the cDNA will be studied. In addition, cDNA clones of the large chromosome will be prepared. The products of this chromosome should form procapsids capable of packaging RNA. The nature of the mechanisms of selective packaging in viruses of segmented genomes is currently unknown. The elucidation of the mechanism for Phi6 will probably be relevant to packaging of other segmented genome viruses.